Chromosome instability (CIN) due to genetic perturbation in cancer cells is now widely recognized to be a major predisposing condition in cancer initiation and/or progression. The general objective of this proposal is to identify and characterize the molecular components required for mitotic chromosome transmission fidelity in yeast and to identify cognate components in mammalian species. The specific aims are: 1). To identify all genes that are mutable to a CIN phenotype, including both essential and nonessential functions. Three independent screens that monitor instability of a chromosomal marker will be used to identify a comprehensive set of proteins that are important for the preservation of genome stability. The corresponding gene sets will be annotated, cross-checked, and compared. 2). To establish genetic interaction maps for CIN genes mutated in human cancer. We will generate comprehensive synthetic lethal genetic interaction maps in yeast for the set of CIN genes altered in human cancer via DNA chip-based screens. RNAi will be used to validate phenotypes in mammalian cells. 3). To establish a resource that systematically cross-references yeast and human CIN genes. Mammalian genes homologous to yeast genes under study will be identified via similarity to yeast protein queries and stored in an annotated searchable database. 4). To investigate the biological functions of NdclO and sumoylation at the kinetochore. Mutations that abrogate sumoylation of NdclO and other kinetochore proteins will be analyzed, and used as starting points for genome-wide screens Further elucidation of the genetic basis of CIN in yeast will provide a mechanistic basis for understanding this process in human cells, and will provide candidate genes for those CIN genes mutated in cancer. Therefore, knowledge gained from this work will provide insight into mechanisms of tumorigenesis.